How can I change the color of the header, behind Page Title?

If you do not use a header image or slider, the header will be a solid color.

Step 1: Go to Appearance > Customize > Colors > Header.

Step 2: Use the Shading Color option to choose the color to fill your header.

If you don't use transparency it will be a solid color. If you use transparency, it will blend with the background color.